(1) Field of the Invention
The present invention relates generally to acoustic performance testing, and more particularly to a method and system for testing the performance of an acoustic treatment in a free-field acoustic environment across a broad frequency range that can include high frequencies up to at least 40,000 hertz.
(2) Description of the Prior Art
Standard acoustic performance testing methodologies describe procedures to assure low frequency acoustic performance. These methods, described in ANSI/ASTM C384-77, present techniques to determine the low frequency cut-off of a material, geometry or shape. Low frequency verification of acoustic performance is experimentally determined using an impedance tube which allows accurate measurement of incident and reflected sound only at low frequencies. However, no high-frequency (i.e., approximately 16 KHz to 40 KHz) acoustic performance testing methodologies exist even though it is important to design and test the performance of materials over the complete bandwidth of interest.
Anechoic chambers are designed to emulate a free-field environment in which all airborne propagating sound energy is absorbed by acoustic treatment installed on the walls of the chamber. The acoustic treatment which is usually wedge shaped, traps an incident sound wave and dissipates the sound energy as heat. In modest size anechoic chambers, the acoustic treatment is required to absorb sound energy from 80 Hz to 40,000 Hz. However, as mentioned above, studies of acoustic absorption material are generally focused on low frequency performance with the assumption that acceptable high frequency performance is inherent in the material selection and is not affected by geometrical design.